A solar cell is roughly classified into a bulk type solar cell and a thin film solar cell. The thin film solar cell is a solar cell in which a semiconductor thin film having a thickness of about several microns is provided as an electric power generation layer on a base material, and its spread in the future is expected as a solar cell with excellent electric power generation efficiency per manufacturing cost (cost performance) because a small amount of semiconductor materials required for the electric power generation is enough as compared with a bulk type solar cell. Above all, compound semiconductor solar cells are expected to be possible to realize a more excellent cost performance because in comparison with thin film silicon solar cells, not only the electric power efficiency is excellent, but a degree of freedom of a manufacturing process is high. In particular, CIS and CIGS solar cells are expected to rapidly spread in the future as a safe solar cell with an excellent cost performance because in comparison with a cadmium (Cd)-tellurium (Te) solar cell that is the same compound semiconductor solar cell, not only excellent electric power generation efficiency is obtained, but cadmium that is a harmful substance is not contained therein (see, for example, Patent Literature 1).
In general, the compound semiconductor solar cell has a structure in which an electrode made of molybdenum or the like, a compound semiconductor layer made of CIS or CIGS or the like, a buffer layer made of CdS or ZnS or the like, and a transparent electrically conductive film made of ZnO, AZO (aluminum-doped zinc oxide) or ITO (tin-doped indium oxide) or the like are formed in this order on a glass base material.